It has for years been recognized that soil erosion occurs as a result of tilling the soil. Normally, great erosion occurs when the tilling is deeper and the soil is turned, as with moldboard plows, which operate at depths of 6 to 10 inches or more. This depth range is referred to herein as "primary tillage" and it is distinguished, for example, from disc tillage and various cultivation techniques such as discs, "spider" wheels and sweeps, as well as combinations of devices which normally cultivate the surface and to nominal depths of two inches, but as deep as 3-4 inches. Despite the possible disadvantage of erosion, some primary tillage is highly desirable to increase the retention and absorption of water; to fracture the soil to promote root development and increase crop yield; and to break up the plow sole (which is a compacted layer of soil found most often just below the working level of some primary tillage implements, such as the moldboard plow).
Farming practices in those areas where the soil undergoes greater erosion have changed substantially over the last few decades. Very little, if any, moldboard plowing is done in these areas. Chisel plows have become popular, particularly over moldboard plows, because they leave more residue on the surface which reduces runoff of surface soil. Mold-board plows turn the soil, burying the residue, and leave the soil on the surface exposed and unprotected from water runoff. Many farmers have changed to conservation farming techniques such as minimum tillage (or "direct seeding" as it is sometimes referred to) in order to reduce soil erosion.
In an effort to reduce erosion, it has become desirable to leave as much of the previous crop residue on the surface as possible, consistent with the farmer's desire to perform some tillage to increase moisture absorption and retention. Residue on the surface not only holds the soil, but also increases moisture retention and promotes seed emergence. This is particularly important in those fields which carry the "H.E.L." designation of the U.S. Department of Agriculture (for "highly erodible land").
The desire to leave crop residue in place on the surface of the soil has been made more difficult by the use of tractors capable of conducting tillage operations at ground speeds of up to seven miles per hour. The shanks of many chisel plows currently available commercially have a thickness of about an inch and a quarter or greater. Because of the thickness, the leading edge of the shank is formed into a tapered wedge to fracture and part the soil. The disadvantage of this type of shank is that when operating at primary tillage depths at ground speeds greater than about two to three miles per hour, the soil and residue at the surface are struck by the shank so that they are thrown to the side, leaving a furrow behind the shank which is wider than the shank. As it rains, the water gathers in these furrows and the furrows become rivulets. The flowing water carries with it the exposed surface soil in the furrows, thereby creating erosion. At higher tractor speeds, the furrow may be 3-4 inches wide and free of any substantial amount of residue that would help reduce erosion.
There are other disadvantages to some current "state of the art" chisel plows, which are, as persons skilled in this art are aware, primary tillage tools. The term "primary tillage" is intended to mean tillage at a depth at least six inches--and up to twelve or fourteen inches--below the surface. In chisel plows having parabolic shanks, as distinguished from the flat, rigid shanks of the present invention, the lifting action of the shank and the point increases the depth and width of the surface furrow formed, and this increases soil erosion. Moreover, many designs of current use impact the soil in an effort to pulverize as much of the soil as possible, like a cultivator sweep, in hopes of increasing moisture retention. However, particularly at speeds which many farmers currently prefer to operate, the effect is to create more fine particles, rather than larger clods; and this tendency further increases erosion because the "fines" are more easily washed away in heavier downpours.